The inefficiency of gene transfer is one of the factors limiting the practice of gene therapy. In some applications, e.g., genetic engineering of early progenitor and stem cells, it has been a major obstacle. Enrichment of a minor population of genetically modified cells by selection of transduced cells provides one avenue, at least in vitro, for overcoming such inefficiencies. One approach for selection involves using a vector in which a gene encoding a selectable product is coupled with a gene encoding a therapeutic protein; selection of cells based on the presence of the selectable product permits the emergence of the subpopulation of cells containing the therapeutic gene (Migita, et al. Proc. Natl. Acad. Sci. USA 92:12075, 1995). Selection may be applied ex vivo (Migita, et al. Proc. Natl. Acad. Sci. USA 92:12075, 1995) or, if a clinically tolerable regimen were devised, repeated cycles of selection might be under-taken in vivo. (See Sorrentino, et al. Science 257:99, 1992). Conventional methods for selection involve the transfer of a gene encoding a product which confers resistance to a cytotoxic drug. Exposure of the cells to the corresponding cytotoxic drug permits selective growth of only those cells transduced with the drug resistance gene.
The clinical applicability of this approach is limited by at least two factors. First, cytotoxic drugs have undesired consequences for the recipient when administered in vivo. Second, the persistent enrichment of genetically modified cells is expected to require that selection be exerted at the level of uncommitted cells. However, selective pressure is very difficult to apply at the level of progenitors and stem cells due to their intrinsic resistance to killing by most cytotoxic agents (Blau, et al. Hum. Gen. Ther. 7:2069, 1996; Allay, et al. Blood 90:3546, 1997).
New methods and materials for selecting a desired subpopulation of cells would be useful in a variety of biological research contexts and could be of particularly great value in clinical applications, especially if the use of cytotoxic drug or other agents with unwanted pharmacologic activities can be avoided.